As for electrophotography, various methods are proposed, as disclosed in JP-B-42-23910 (The term "JP-B" used herein means an "examined Japanese patent publication"), JP-B-43-24748, and US Patent No. 2,297,691. However, generally used is such a method that an electrostatic latent image is formed on the surface of a photoconductor made of photoconductive material, the latent image is developed with a toner, and then the toner is transferred onto an image support such as paper or the like.
On the other hand, as for a method of transferring the toner image formed on the photoconductor onto the image support, there is a method using a transfer belt. According to this method, the image support is transported by the transfer belt while being electrostatically attracted thereto at the step of transferring the toner image formed on the photoconductor directly onto the image support so as to form an image. It is therefore possible to eliminate a support pressing mechanism which is provided for the purpose of preventing the image support from being transported meanderingly, or the like. Accordingly, there is an advantage that a printing prohibition area is considerably reduced, so that an image can be formed over the whole are of the image support.
In the above-mentioned electrophotographic system using a transfer belt, various properties are required for transferring a toner image uniformly and efficiently from a photoconductor onto an image support. For example, an elastic layer is provided on the surface of the transfer belt that is located at the side of the image support opposite to the photoconductor side, and such a mechanism that absorbs stress from the outside to some extent is provided. Then, the surface of the photoconductor is prevented from being damaged when a foreign matter such as a carrier is inserted in a nip portion between the photoconductor and the image support. It is effective in prolonging the life of the photoconductor. This method has another advantage that the nip can be ensured stably at a transfer point between the photoconductor and the image support. In addition, the transfer belt is required to have suitable electric properties (resistance, electrification, electric current flowing upon transferring, etc.) in which a toner image can be transferred onto the image support uniformly while preventing disorder of the image, or to have mechanical properties in view of the life of the transfer belt itself. Therefore, the transfer belt itself is constituted by numbers of layers so that some of the layers are made to have electrical conductivity, and some of the layers are improved in mechanical strength to thereby adjust the balance of the respective properties.
However, there was a problem that a conventional transfer belt lacked stability in transporting the image support even if elasticity was given to the transfer belt, or even if electric conductivity and mechanical properties thereof were adjusted as mentioned in the above conventional techniques.
That is, when a new transfer belt was attached to form an image, a phenomenon called "drum wrap" in which an image support wrapped on a photoconductor sometimes occurred at an initial stage. Further, not only at an initial stage, but also in any other times, there occurred a problem that the image support meandered, or the reliability of an image forming apparatus itself was lowered. In addition, when these problems were intended to be improved, toner scattering or image dislocation such as transfer failure was caused at the time of transfer. This problem has a trade-off relation with respect to the above-mentioned problems.
Further, there was another problem that the characteristics of paper used by users were largely different depending on the users so that it is very difficult to ensure the stable transport of the paper and the image quality to be obtained.
Particularly recently, an electrophotographic apparatus is required so as to have features such as high speed, high image quality, power saving, and low cost, and severely required so as to have properties of stability in transporting an image support, maintenance-free of the apparatus, and stability in image quality.